Hopefully you’ve seen the recent blog post by my colleague Seyed Saddoughi on the ECO ROTR, explaining the concept’s origins and 1:50 scale testing. Given the potential of the technology, GE wanted to move quickly to prove that it works at full scale. Starting in 2013, I was called upon to lead the development of a full-scale prototype.
This project was my first exposure to working on wind turbines, and it made me appreciate the level of engineering that goes into these seemingly simple machines. Unlike gas or steam turbines that are designed to operate under a relatively limited number of set conditions, wind turbines must operate reliably and safely under literally hundreds of conditions, many of them highly transient. A thorough understanding of the aero-structural coupling of these machines is crucial, under varying wind speeds and directions, sudden gusts, and storms. The controls are quite sophisticated, continually yawing the machine into the wind and pitching the individual blades to the optimum angle for the incoming wind velocity. The blades are long and flexible, as is the tower, and the dynamic coupling of these components is an important design consideration. So installing the ECO ROTR dome – which is 20 meters in diameter and weighs 20 tons — on the hub of a real, operational wind turbine presented a significant challenge. The team had to make sure that the wind turbine was properly adapted to handle all the effects of the dome under all operating conditions.
It is worth mentioning that the machine chosen for the ECO ROTR test is itself a prototype – the 1.7 MW, 100-meter diameter space frame tower (SFT) machine at Tehachapi, California. This is a GE-owned machine that can be used to test new hardware before the technology is offered commercially to GE customers. To a great extent, the timing of the dome installation was dictated by the schedule for testing other components by the GE Wind business. But it also enabled the possibility of sharing some of the peripheral costs (such as cranes) with other test campaigns.
Early on, it was decided that the prototype dome would be a geodesic construction as shown in Figure 1. The reason is simply that it was the construction method that required the least amount of unknown risk. A vendor was identified that had extensive experience designing and building large geodesic-type structures, although they had never built one that was required to rotate while positioned vertically on a 97-meter-tall tower!
The GRC team in Niskayuna undertook the detailed design of the mounting adapters (Figure 2), and all the attachment hardware. Turbine and tower loads were calculated for all the relevant normal (and abnormal) operating conditions, including variations in wind speed and direction, storms, gusts, and instances where the rotor or blades are not positioned optimally for the wind that the turbine is experiencing, and these were used in the design of the new hardware as well as the evaluation of the machine’s existing components. In addition to enhancing the turbine’s performance, the dome also increases the loads on the machine due to both the added mass and the aerodynamic pressure on the dome surfaces. Jim Madge and Ian Wilson from GE Wind were instrumental in helping the team evaluate the safety margins on existing components as well as the design of the dome and mounting hardware.
As one might imagine, the number one requirement for the prototype was the safety of the space frame tower turbine and personnel. So the design of the dome structure, mounting adapters, brackets, and bolting went through several iterations as the team refined the predicted load conditions that the prototype hardware would see during its brief service life of four months. The four-month test duration was chosen because it is sufficiently long to collect data under all the necessary wind conditions (remember, this isn’t a fossil-fueled machine where the operator can just dial in a setting; we have to wait for the wind to cooperate). At the same time, a permanent structure would require more design robustness than is acceptable for a short-term prototype. Even so, the installation was required to follow a formal permitting process with the local county government agencies, necessitating a thorough review of the entire turbine structural design by a licensed Professional Engineer.
The dome and hardware were delivered and assembled at the Tehachapi site in 2014. The attached video, shot from the nacelle of the neighboring SFT turbine, shows the dome assembly, overseen by Garth Nelson. This was followed by a long delay while the turbine was used for other component tests by the Wind business. As it turned out, this was time that the team needed to work through all the details of the controls modifications required to operate the turbine with the dome. As mentioned earlier, the dome’s presence changes the dynamic behavior of the machine and this must be compensated by the controller. In addition, normally the yaw angle of the rotor would be established based on the wind velocity measured by an anemometer on top of the nacelle; however, with the dome blocking the wind from the sensor, a new anemometer was installed on the meteorological mast (met mast) approximately 250 meters upwind and slightly offset from the turbine.
For safety reasons, the dome was actually assembled about 300m from the turbine and needed to be transported to the base of the turbine when the time came for installation. Figure 3 shows the dome being carried by a crane to its point of “lift-off.”
When it was time for the actual dome installation, things did not go entirely smoothly. Despite using a specially-made template to ensure that the dome adapter holes would align with the bolt holes on the dome itself, after the adapters were mounted to the hub it was discovered that bolt circle diameter was approximately 8mm too small to fit the dome, apparently due to dimensional differences between the hub on the machine and the one used in the factory to fabricate the template. The team learned and adapted. Custom shims were made to adjust for the error, and the adapters were adjusted into their proper positions. Figure 4 shows the adapters in place with the alignment template installed, indicating that proper alignment had been achieved.
On Memorial Day 2015, the winds cooperated and the dome went up. The team’s hard work was rewarded, and one of the field engineers on site captured the event using a drone-mounted camera, including the removal of the rigging by one of the brave field engineers (Figures 5, 6 and 7).
Now begins a lengthy test procedure to evaluate the effect of the dome on aerodynamic performance, turbine loads, and noise, which will be covered in future blogs. Read about the dome installation on GE Reports.
Global team, outstanding effort
In my 25+ years of working at GE, I have never worked on a more global, and cohesive, team as the one assembled for the ECO ROTR project. This was a textbook collaboration, with the GRC folks from Niskayuna leading the dome structural design and hardware fabrication, Bangalore leading the Computational Fluid Dynamics and aerodynamics analyses and Munich leading the aeromechanics and loads aspects. Certain tasks, such as the controls, instrumentation and data collection, required involvement from staff at all three sites. On top of that, the team had very strong support from GE Wind folks in Greenville and Schenectady plus representation in the field at the prototype test site in Tehachapi, California. I’d like to mention the names of all the outstanding people involved with the project. Together, we made ECO ROTR real!
Dome Structure Design & Fab: Fulton Lopez, Garth Nelson, Dan Erno, Bob Zirin, Zaeem Khan, Peggy Baehmann, James Simpson
Aerodynamics: Jaikumar Loganathan, Anindya Sengupta, Srinath Narayana Murthy, Sachin Premasuthan, Mark Braaten, Dominic von Terzi
Aeromechanics & Loads: Sara Delport, Marion Reijerkerk, Thomas Merzhaeuser, Mike Moscinski
Controls and Instrumentation: Andreas Herrig, Samuel Davoust, Stefan Kern, Matt Boespflug, Pat Riley, Dmitry Opaits
GE Wind: Jim Madge, Ian Wilson, Bill Holley, Brandon Gerber, Jignesh Gandhi, Sven Hansch, Todd Andersen, Greg Cooper, Tim McCorkendale, Amos Taliulu, Patrick Stevenson, Juan Avila, Howard Hansen